Lithium ion secondary batteries, which have features such as high output, high energy density and light weight, are employed in various applications, and studies of such batteries are still vigorously being conducted. Lithium ion secondary batteries include a non-aqueous electrolyte containing a non-aqueous solvent, and electrodes having a high energy density. Accordingly, a further improvement in safety has been an important issue for lithium ion secondary batteries.
In order to insure a high level of safety, it is necessary to suppress an increase in battery temperature or the like in battery tests performed under extremely severe conditions such as a nail penetration test and a crush test. In the tests, if a material mixture is detached from a current collector, forming an exposed portion of the highly conductive current collector, a possibility increases that a large current may flow or excessive heat generation may occur.
Conventionally, attempts have been made to insure safety by using an electric circuit including a fuse or a PTC element, by utilizing the shut-down function of a separator, by forming a resistance layer for increasing resistance under severe conditions on a current collector surface, or the like, but these methods do not provide sufficient safety (see Patent Document 1). Particularly when a resistance layer is formed on a current collector, the internal resistance of the battery increases even under normal use, causing a problem of low output.    Patent Document 1: Japanese Laid-Open Patent Publication No. Hei 10-199574